Electrostatic discharge (ESD) may occur during packaging, testing, and transporting of integrated circuits (ICs) due to the electrostatic charge accumulated on human body or instruments, lead to unrecoverable damage to semiconductor elements in ICs, and affect the functionality of ICs.
Therefore, to avoid the damage caused by ESD, ESD protecting circuits are required in ICs. Generally, an ESD protecting circuit includes at least one high voltage semiconductor device, such as lateral diffused N type metal oxide semiconductor transistor (LDNMOS). The maximum withstanding voltage is the most important concern of high voltage semiconductor devices. As the dimensional size of semiconductor devices is becoming smaller and smaller, it is more and more difficult to improve the maximum withstanding voltage.